1. Field of the Invention
The present invention relates to a styrenic resin molding and a process for producing the same. More particularly, it pertains to a styrenic resin molding having a high crystallinity and excellent in heat resistance, solvent resistance and transparency, and a process for efficiently producing the same.
2. Description of Related Art
There has heretofore been increasingly desired a container endowed with all of heat resistance, oil resistance and transparency mainly as an alternative to a glass container. As a general rule a container made of polyethylene terephthalate, polypropyrene, polyethylene, polystyrene or the like each being a thermoformed molding has been insufficient in heat resistance, thus failing to meet the above-mentioned desire.
On the other hand, a styrenic polymer having syndiotactic configuration is known to be high in heat and solvent resistances and is expected to find a variety of applications. In order to make full use of the characteristics of the aforestated styrenic polymer, however, it is necessary that the molding made of the styrenic polymer be provided with a sufficiently high crystallinity. Nevertheless, the molding of a syndiotactic styrenic polymer crystallized by the conventional method is known to be low in transparency.
As an example of molding having transparency which is made of a styrenic polymer having syndiotactic configuration, there are known a sheet with a low crystallinity and a film with refined crystal by means of orientation (refer to Japanese Patent Application Laid-Open Nos. 168709/1989 and 316246/1989). However, the sheet with a low crystallinity is insufficient in heat and solvent resistances, and although it can be thermoformed, the end thereof is apt to whiten, thus making it difficult to produce a wholly transparent container. In addition in spite of its extremely excellent properties, the aforesaid oriented film is expensive because of the costly orientation equipment to be used therefor; besides in the case of producing a thick oriented film, it is necessary to increase the thickness of the original sheet with a low crystallinity before orientation, thus making it difficult to produce a molding having a thickness of 300 .mu.m or more. Moreover, the highly transparent oriented film is insufficient in thermoformability.
There is also known a method for heat-treating a styrenic polymer having a low crystallinity (Japanese Patent Application Laid-Open No. 272608/1989), which method however, has not necessarily been successful in forming a transparent and crystallized molding.
In view of the aforestated circumstances facing such difficulty, in order to develop a highly crystallized and transparent molding capable of coping with the requirement of a wide range of thickness, especially a thickness of 300 .mu.m or more, intensive research and investigation were made by the present inventors on a method for controlling the growth of both the positively birefringent crystal in the primary crystallization and the negatively birefringent crystal in the secondary crystallization, thus suppressing light scattering. As a result it has been discovered that a highly transparent thermoformable or orientable non-oriented molding having a high crystallinity can be efficiently produced by subjecting a preform for heat treatment having a crystallinity of 20% or less which comprises as the principal component a styrenic polymer having syndiotactic configuration to high-speed heating and heat treatment within a definite temperature range and that the objective molding further excellent in transparency can be efficiently produced by subjecting the non-oriented molding thus obtained to thermoforming and orientation treatment, which molding being exemplified by wholly transparent and heat-resisting containers, transparent and heat-resisting sheets having a thickness of 300 .mu.m or more, etc. The present invention has been accomplished on the basis of the above-described finding and information.